The invention concerns an electrical circuit for a device for measuring fluid levels in industrial tanks and the like. It has an electronic transmitter and receiver for microwaves and a cavity resonator equipped with an aperture and antenna in which the signal reflected from the liquid surface and the emitted signal are fed to a frequency conversion stage. Its low-frequency output signal is fed via an analog-to-digital converter to a microprocessor that calculates the fill level.
Level measurement devices utilizing microwaves are known. They transmit signals in the microwave range which are reflected on the surface of the medium in the tank and received by a receiver. The distance between the level measurement device and the surface of the medium is determined from the signals by means of an electronic circuit which includes microprocessors and electronic computers. When such microwave level measurement devices are used with tanks, like industrial tanks, subject to an underpressure or overpressure at high or low operating temperatures, especially when they contain explosive and/or corrosive and/or toxic media, it is necessary to separate the tank interior from the electronic transmitting and receiving part. For this purpose a cylindrical cavity resonator aperture made of quartz glass or the like is arranged in the waveguide or cavity resonator protruding through the top of the tank, this aperture advantageously having a low dielectric loss factor for microwave transmissions. In level measurement devices of this type a transmitted signal that continuously varies in frequency is generated by a voltage-controlled oscillator and directed toward the liquid surface via the waveguide provided with an aperture and antenna. The electromagnetic waves reflected by the liquid surface are received by the antenna and converted by the main part into an electrical frequency signal that is fed to a frequency converter. Owing to the level-dependent travel time of the microwaves in the tank the incoming signal undergoes a frequency shift relative to the emitted signal that is directly proportional to the level.
A frequency conversion stage converts the emitted and incoming signals to a low-frequency signal that is digitized and processed in the microprocessor. However, the optical aperture arranged in the waveguide disadvantageously causes interference signals by reflection of the microwaves, the amplitudes of which are much greater than those of the useful signal. In practice the output signal of the frequency conversion stage owing to any mode changes of the microwave signal and/or multiple reflections of the optical aperture contains a power density fraction that can be much higher than the fraction of the useful signal reflected from the liquid surface. The spectral lines of the power density spectrum that occur at different filling heights yield relatively high values at low frequencies that are due to reflection of the optical aperture, but lower values at higher frequencies as a result of the longer measurement path. The high low-frequency power fractions that occur, especially when the optical aperture is contaminated, are also present at low liquid level. The deviations between the low-frequency and high-frequency power density values can lie between 4- and 20-fold in industrial tanks and the like or even higher.
DE-OS 3,134,243 discloses a circuit for a level measurement device without a cavity resonator aperture, in which a filter is arranged between a frequency conversion stage and an A/D converter, which is intended to filter out undesired frequencies. Since distance information is part of the mixed frequency, filtering out undesired frequencies would also filter out the corresponding useful signals and not permit distance determination in this frequency range at all.